ULTRAPURE HEXAVALENT CHROMIUM TREATMENT FOR RESIDENTIAL SUPPLY

摘要

With a long and historic presence in the community of Hinkley, CA, the Pacific Gas & ElectricrnCompany (PG&E) is committed to addressing issues associated with hexavalent chromiumrn(Cr(VI)) in the local aquifer. In addition to an ongoing groundwater remediation program,rnPG&E has been engaged in a groundbreaking effort to provide local area residents with wellrnwater treated to unprecedented levels. In June 2012, the state of California’s Lahontan RegionalrnWater Quality Control Board (Water Board) required PG&E to provide replacement drinkingrnwater for homes served by domestic or community wells in a defined area with total and/orrnCr(VI) concentrations exceeding 3.2 and 3.1 mg/L, respectively. Although Cr(VI) is anrnimportant emerging contaminant, at the time of the Water Board order, no state had promulgatedrna drinking water standard. However, California had established a very low public health goal PHG) of 0.02 mg/L, a theoretical benchmark based on eliminating any health risk without regard to technical or economic feasibility. Thus, in the absence of an enforceable maximum contaminant level (MCL) for potable water, the Water Board mandated that PG&E utilize the PHG as the treatment standard for its Whole-House Replacement Water (WHRW) Program. Because even the most sensitive analytical methods for Cr(VI) detection were not considered reliable for concentrations as low as 0.02 mg/L, the Water Board allowed an effective standard equivalent to the method reporting limit (MRL) of 0.06 mg/L. Treating Cr(VI) to a standard of 0.06 mg/L for drinking water applications had never been attempted, and even tests of off-the-shelf bottled water have exhibited Cr(VI) concentrations exceeding the 0.06 mg/L WHRW Program standard. Moreover, California’s 10 mg/L MCL, subsequently proposed in August 2013 and finalized in July 2014, is three orders of magnitude higher than the PHG standard applied to the WHRW Program.rnWorking with PG&E, ARCADIS conducted pilot testing on various treatment technologies to identify an innovative ion exchange (IX) process not only capable of achieving the 0.06 mg/L standard, but also easily replicable for multiple sites and able to accommodate widely varying water quality from more than 30 different residential wells. Despite the flexibility and robustness of the subsequently designed mobile treatment systems, this unique challenge was compounded by an ARCADIS study showing the potential for trace level Cr(VI) leaching from sources downstream of the system’s IX process, including metal components, household plumbing, chlorine tablets, and other materials with NSF-certification for potable use.rnThe purpose of this paper is to discuss the ultrapure treatment process utilized to successfully achieve the benchmark 0.06 mg/L level for Cr(VI) in potable water, along with the significant challenges associated with materials leaching in the context of a parts-per-trillion standard.